This invention relates to improvement of a line pressure control unit of an automatic transmission including a continuously variable transmission.
An automatic transmission comprising a continuously variable transmission such as a toroidal continuously variable transmission and a V belt continuously variable transmission for vehicles, controls speed change by oil pressure, and controls a line pressure which is an oil pressure acting on an oil pressure circuit, as a base pressure.
Tokkai Hei 8-324297 published by the Japanese Patent Office in 1996 discloses a control method wherein a line pressure a required line pressure for the automatic transmission is multiplied by a constant safety factor to calculate a target line pressure.
Further, Tokkai Hei 6-207567 published by the Japanese Patent Office 1994 discloses a method wherein a real line pressure is feedback controlled to the above-mentioned target line pressure.
The idea of performing feedback control of the line pressure by multiplying the required line pressure for control of an automatic transmission by a constant safety factor to give a target line pressure, can easily be inferred from the two above-mentioned examples of the prior art.
When setting the safety factor as a fixed value to determine the target line pressure, the setting of the safety factor must be based on the maximum value of variation. However, with such a setting, the safety factor becomes a large value and this causes a pump drive load to increase under feedback control.
It is therefore an object of this invention to provide a line pressure control unit for an automatic transmission for a vehicle which controls a line pressure so that the pump drive load is small under feedback control.
In order to achieve above object, this invention provides a line pressure control unit of an automatic transmission for a vehicle which performs feedback control of a line pressure of an automatic transmission, comprising: a line pressure solenoid valve which adjusts a line pressure, a sensor which detects a real line pressure, and a microprocessor which controls the line pressure solenoid valve, the microprocessor being programmed to: calculate a required line pressure according to the running state of the vehicle , calculate a target line pressure from the required line pressure, the target line pressure being modified by a safety factor for line pressure correction, and calculate a line pressure difference between the required line pressure and the real line pressure, and add a feedback control amount according to the line pressure difference to the target line pressure to calculate a command line pressure, control the line pressure valve based on the command line pressure.